OBJECTIVE: To identify communication needs and evaluate the effectiveness of alternative communication strategies for bioterrorism responses. METHODS: We provide a framework for evaluating communication needs during a bioterrorism response. Then, using a simulation model of a hypothetical response to anthrax bioterrorism in a large metropolitan area, we evaluate the costs and benefits of alternative strategies for communication during a response. RESULTS: Expected mortality increases significantly with increases in the time for attack detection and announcement; decreases in the rate at which exposed individuals seek and receive prophylaxis; increases in the number of unexposed people seeking prophylaxis; and increases in workload imbalances at dispensing centers. Thus, the timeliness, accuracy, and precision of communications about the mechanisms of exposure and instructions for obtaining prophylaxis and treatment are critical. Investment in strategies that improve adherence to prophylaxis is likely to be highly cost effective, even if the improvement in adherence is modest, and even if such strategies reduce the prophylaxis dispensing rate. CONCLUSIONS: Communication during the response to a bioterror attack must involve the right information delivered at the appropriate time in an effective manner from trusted sources. Because the response system for bioterror communication is only fully operationalized once an attack has occurred, tabletop planning and simulation exercises, and other up-front investments in the design of an effective communication strategy, are critical for effective response planning.